This invention relates to a method of operating a plasma addressed liquid crystal display panel.
Several different types of raster scan liquid crystal (LC) display panels have been developed. One type is known as the active matrix addressed LC display panel and another type is known as the plasma addressed LC display panel. Each type of panel comprises a rectangular array of cells. The various types of LC display panels differ with respect to the manner in which the individual cells are addressed in order to display an image.
FIG. 3 illustrates schematically a single cell 40 of an active matrix addressed raster scan LC display panel. As shown in FIG. 3, this cell comprises upper and lower electrodes 44 and 48 having a layer 52 of twisted nematic (TN) liquid crystal material therebetween. The LC material is almost a perfect insulator, and accordingly the electrical behavior of the cell is similar to that of a capacitor. The upper, or column, electrode 44 is connected to the output terminal of an amplifier 54 which provides a positive voltage that depends upon the desired state of the cell 40. The lower, or row, electrode 48 is connected to ground through a transistor 56. In order to address the cell, the transistor 56 is turned on, forcing the electrode 48 to a potential near ground. The capacitor is charged to a voltage that depends on the output voltage of the amplifier 54 and a field exists in the layer of liquid crystal material. If the voltage between the row and column electrodes is zero, the cell is off, whereas if the voltage is, for example, +3 volts, the cell is on. The voltage may have an intermediate value in order to establish an intermediate state.
The cell 40 is addressed once per frame of an incoming raster scan video signal. Thus, the output voltage of the amplifier 54 is set at a level that depends on the voltage of the video signal during a sampling interval, the transistor 56 is turned on in order to charge the capacitor, and the transistor 56 is then turned off. One frame interval later, the same procedure is repeated.
If the transistor 56 had no off current, the voltage between the row and column electrodes would remain constant until the transistor was turned on again one frame later. However, the transistor 56 has a finite off current, and accordingly the voltage declines during the interval after the transistor turns off. (The liquid crystal material is not a perfect insulator, and accordingly conduction by the LC material contributes to the discharging of the capacitor. Nevertheless, the time constant for the discharge of the capacitor due to conduction by the LC material is much longer than the frame period of the video signal whereas the time constant for discharge due to the off current of the transistor can be shorter than the frame period, and so the off current of the transistor 56 can be the dominant mechanism for discharge of the capacitor.)
The plasma addressed liquid crystal (PALC) display panel shown in FIG. 4 comprises, in sequence from below, a polarizer 2, a channel substrate 4, a cover sheet 6 (commonly known as a microsheet), a dyer 10 of TN liquid crystal material, an array of parallel transparent data drive electrodes (only one of which, designated 12, can be seen in the view shown in FIG. 2), an upper substrate 14 carrying the data drive electrodes, and an upper polarizer 16. The channel substrate 2 is typically made of glass and is formed with multiple parallel channels 20 in its upper main face. The channels 20 are filled with an ionizable gas, such as helium. A cathode 24 and an anode 26 are provided in each of the channels 20. The channels 20 are orthogonal to the data drive electrodes and the region where a data drive electrode crosses a channel (when viewed perpendicularly to the panel) forms a discrete panel element 26. Each panel element can be considered to include elements of the layer 10 and the upper and lower polarizers 2 and 16. In the case of a color display panel, the panel elements include color filters (not shown) between the layer 10 and the upper substrate 14. The region of the upper surface of the display panel that bounds the panel element constitutes a single pixel 28 of the display panel.
As explained in U.S. Pat. No. 5,077,553, when a suitable potential difference is established between the cathode and anode in one of the channels, the gas in that channel forms a plasma that provides a conductive path at the lower surface of the cover sheet 6. If the data drive electrode is at ground potential, there is no significant electric field in the volume element of TN liquid crystal material and the panel element is considered to be off, whereas if the data drive electrode is at a substantially different potential from ground, there is a substantial electric field in that volume element of liquid crystal material and the panel element is considered to be on. An extended light source (not shown) is provided beneath the panel. In the event that a panel element is off the upper polarizer passes light received from the volume element of liquid crystal material and the panel element is illuminated, whereas if a panel element is on, the upper polarizer blocks light received from the volume element of liquid crystal material and the pixel is not illuminated. Black surround material (not shown) is provided between adjacent panel elements in order to absorb stray light and preserve maximum contrast between a panel element that is on and a panel element that is off.
In the case of a plasma addressed liquid crystal display panel, the off current (the rate at which charge is supplied to a panel element via circuitry outside the capacitor) is approximately zero, and consequently, the time constant for decay of the electric field in a panel element is considerably longer than the frame period of the video signal.
The conditions that may exist in a channel of a PALC display panel when a plasma is present may limit the useful life of a PALC display panel. The time for which the plasma exists in a channel depends on the operating conditions of the panel. If the time could be reduced, the useful life of the panel may be extended.
The useful life of a PALC display panel depends at least in part on the amount of time for which a plasma exists in a given channel. Thus, although it is widely accepted that a display panel must have a useful life of about 10,000 hours in order to be commercially acceptable, in the case of a display panel driven by a video signal at a frame rate of 60 Hz and having 480 active lines per frame, over 10,000 hours of operation a plasma exists in a given channel only for about approximately 3-4 hours.